A term authors (or any self-employed person) love to throw around is "word of mouth." That golden moment when books just start to sell themselves because they have taken on a life of their own.
But is it really "word of mouth"? I mean, be honest, how often do you bring up books in daily conversation? Do you highly recommend to your friends every book you really enjoyed? I'm sorry to say that I don't. Lots of things factor into a book recommendation. For one thing, I have to think about it at the time. For another, the friend or family member I'm talking to has to share the same taste in books.
Way back when, people used to talk about books the way we talk about TV shows. "Word of mouth" was definitely how an author got exposure. But I would say the more accurate term these days is "word on the internet." Sad, but true. I would argue that an author can be extremely successful by simply being an internet presence alone and never having a single live person recommend their book to another live person.
From a marketing perspective, this is very important to keep in mind. Everything that you do online as an author is your modern day "word of mouth." Of course, live referrals do certainly help things. But it's important to realize that you're not just an internet profile you can hide behind.